1. Field of the Invention
This invention relates to variable resistance circuits comprising electronic circuits, and more particularly to variable resistance circuits controllable by a digital code signal or a pulse-width signal.
2. Description of the Prior Art
Prior art variable resistance circuits comprise a transistor or a field effect transistor (FET) with an equivalent resistance present between the collector and the emitter of the transistor or between the drain and the source electrodes of the FET, which is controlled by an input signal supplied to the base of the transistor or to the gate of the FET, whereby the resistance of the circuit is varied to a desired value. These prior art circuit leave much to be desired. The prior circuits simply utilize the characteristics of the transistor or FET, have a limited resistance-variable range and are incapable of offering a simple response (e.g. square characteristic response or 3/2-power characteristic response) to the control signal. Furthermore, the control signal is of an analog voltage or current and hence is not completely compatible with digital logic circuits.